In both model and biological membranes, lipids have the capability of existing in lateral domains which differ from other lateral groupings in composition. However, the arrangement of phospholipid molecular species within biological membranes is not well understood. In the proposed work, the chemical composition of lipid phases and lipid domains in synthetic lipid mixtures, isolated Escherichia coli lipids, and the intact cytoplasmic membrane of E. coli will be investigated. Fluorescence polarization of parinaroyl phospholipids will be used to assess the lipid phases present, while chemical crosslinking of nearest neighbor lipids will be used to determine the lipid arrangement of the membrane fluid phase lipids. Phosphatidylethanolamine species will be crosslinked by the amine-specific reagent, dimethylsuberimidate, and the products analyzed by high pressure liquid and gas-liquid chromatography. Lipid species, mapped to a phase and domain in intact membranes, will be localized to the inner or outer monolayer using the nonspecific lipid transfer protein from bovine liver and by chemical labelling with trinitrobenzenesulfonic acid. These studies will examine the membranes of E. coli with mutations in fatty acid metabolism. Grown with various fatty acid supplements, the membranes of these cells develop functional defects due to their lipid compositions. The chemical and physical studies described here will be integrated into a more clearly defined model of membrane structure, which, in the long term, will provide a basis to relate membrane lipid topology to membrane function.